1. Field of the Invention
The present invention relates to a long term evolution (LTE) system and, more particularly, to a mobile communication system for differently performing radio resource reconfiguration according to types of handover, and a method for processing a handover procedure thereof.
2. Discussion of the Related Art
FIG. 1 shows a network structure of the E-UMTS, a mobile communication system, applicable to the related art and the present invention. The E-UMTS system has been evolved from the UMTS system, for which the 3GPP is proceeding with the preparation of the basic specifications. The E-UMTS system may be classified as the LTE (Long Term Evolution) system.
The E-UMTS network may be divided into an evolved-UMTS terrestrial radio access network (E-UTRAN) and a core network (CN). The E-UTRAN includes a terminal (referred to as 'UE (User Equipment), hereinafter), a base station (referred to as an eNode B, hereinafter), a serving gateway (S-GW) located at a termination of a network and connected to an external network, and a mobility management entity (MME) superintending mobility of the UE. One or more cells may exist for a single eNode B.
FIGS. 2 and 3 illustrate a radio interface protocol architecture based on a 3GPP radio access network specification between the UE and the base station. The radio interface protocol has horizontal layers comprising a physical layer, a data link layer, and a network layer, and has vertical planes comprising a user plane for transmitting data information and a control plane for transmitting control signals (signaling). The protocol layers can be divided into the first layer (L1), the second layer (L2), and the third layer (L3) based on three lower layers of an open system interconnection (OSI) standard model widely known in communication systems.
The radio protocol control plane in FIG. 2 and each layer of the radio protocol user plane in FIG. 3 will now be described.
The physical layer, namely, the first layer (L1), provides an information transfer service to an upper layer by using a physical channel. The physical layer is connected to an upper layer called a medium access control (MAC) layer via a transport channel, and data is transferred between the MAC layer and the physical layer via the transport channel. Meanwhile, between different physical layers, namely, between a physical layer of a transmitting side and that of a receiving side, data is transferred via a physical channel.
The MAC layer of the second layer provides a service to a radio link control (RLC) layer, its upper layer, via a logical channel. An RLC layer of the second layer may support reliable data transmissions. A PDCP layer of the second layer performs a header compression function to reduce the size of a header of an IP packet including sizable unnecessary control information, to thereby effectively transmit an IP packet such as IPv4 or IPv6 in a radio interface with a relatively small bandwidth.
A radio resource control (RRC) layer located at the lowest portion of the third layer is defined only in the control plane and handles the controlling of logical channels, transport channels and physical channels in relation to configuration, reconfiguration and release of radio bearers (RBs). The radio bearer refers to a service provided by the second layer (L2) for data transmission between the UE and the UTRAN.
A random access channel (RACH) will now be described. The RACH is used to transmit data with a relatively short length to uplink, and in particular, the RACH is used when a UE, which has not been allocated dedicated radio resources, has a signaling message or user data to be transmitted to uplink. Or, the RACH may be also used for a base station to instruct a UE to perform a RACH procedure.
The mobile communication system is different from a fixed line communication system in that the mobile communication system should provide a seamless service to UEs with mobility. Namely, the mobile communication system features that it supports the case where a UE moves to a different area. When the UE becomes away from a connected base station and simultaneously becomes close to a different base station, a network should perform an operation of changing a connection point of the UE to the new base station from the serving base station. While the UE is disconnected from the old base station and completes its connection to the new base station, there is no data transmission or reception.
Meanwhile, every user data has a transmission limit time. For example, in case of a voice call, single voice information generated from a device intending transmission of the voice information should be transmitted to a device that is to receive the voice information within a certain time. In addition, data such as TCP ( ) should be delivered from a sender to a recipient within a certain time, and the reception side should inform the sender about acknowledgement of transmission/reception of the TCP data within a certain time, otherwise the sender would re-transmit the TCP data.
In general, the UE and the base station continuously exchange transmission/reception acknowledgement information with respect to the transmitted/received data. For example, in case of a TCP packet, if a single packet is lost by lower entities while being transmitted from the sender to the recipient, the transmission side TCP entity rapidly lowers a transmission rate (speed) of the TCP data. For example, the transmission side TCP entity, which generates data and transmits the same to the reception side at a rate of 100 Mbit/s, determines that the reception side has failed to receive even only one packet, the transmission side TCP entity rapidly lowers the transmission rate of the TCP data to, for example, 10 Kbit/s.
Thus, in the mobile communication system, in order to reduce an influence on the TCP, a lossless mode to effectively support traffic such as the TCP between the base station and the UE. The lossless mode may be considered an AM (Acknowledge Mode) RLC, and if the transmission side fails to receive a reception acknowledgement response with respect to its transmitted data from the reception side within a certain time or if the transmission side receives information of a reception failure with respect to its transmitted data, the transmission side re-transmits the data.
In this case, however, when the transmission side receives the information of reception failure with respect to certain data, it does not always perform retransmission with respect to the corresponding data but performs retransmission only when a transmission/reception acknowledgement response is made within a maximum transmission delay time defined in a radio interface.
FIG. 4 is a signal flow chart illustrating a handover procedure between the UE and the base station defined in the LTE.
As shown in FIG. 4, the UE performs measurement on the strength or the like of signals with respect to each cell, and if a particular reference (base) designated by the base station is satisfied according to the measurement results, the UE transmits a measurement report message to a source base station (source eNB) via uplink (UL) (S10).
The source eNB determines performing of handover to move the UE to a cell of a target eNB with reference to the measurement report message received from the UE, and transmits context data to the target eNB to request preparation of handover (S20).
The target eNB secures radio resources under its management, and transfers radio resource configuration information together with a temporary identifier (new C-RNTI) with respect to the corresponding UE to the source eNB. The source eNB transmits a handover command to the UE (S40) and then starts transmission of user data (PDCP SDU) or the like to the target eNB. In this case, the source eNB transmits data (PDCP SDUs) which have been successively received from the UE to a core network (MME/UPE), and transfers first one (PDCP SDU) of data which have not been successively received from the UE, to the target eNB. Also, the source eNB delivers data which have not been acknowledged by the UE, among the data (PDCP SDUs) the source eNB had transmitted to the UE.
Upon receiving the handover command, the UE reestablishes a radio environment with the target eNB including timing synchronization (S50). The target eNB response to the UE by timing information, and the UE transmits a handover confirmation message to the target eNB (S60). In this case, the target eNB transmits reception acknowledgement information with respect to the handover confirmation message to the UE. Additionally, the target eNB may transmit user data transmission/reception confirmation information (PDCP status report) to the UE. The PDCP status report informs about which user data (PDCP SDUs) have been successfully received by the source eNB from the UE and about which user data (PDCP SDUs) have not been successfully received. In other words, the PDCP status report may be interpreted as information of user data the target eNB requests its retransmission from the UE.
Upon receiving the PDCP status report, the UE retransmits data informed to have not been successfully received by the source eNB among the data attempted to have been transmitted to the eNB, and does not retransmit data informed to have been successfully received by the eNB.
Thereafter, when the handover is completed, the target eNB informs the source eNB about the success of handover (S70), and transmits user data to the core network (MME/UPE) to update the location of the UE. At this time, with respect to the user data which had been attempted to be transmitted by the UE when the UE located within the source eNB, the target eNB performs realigning on the user data received from the source eNB and the data received from the UE located within the target eNB, and transfers successfully restored user data to the MME/UPE.
In the related handover procedure, as soon as the UE is connected to the new target eNB, the UE starts transmission to the target eNB, starting from the first user data which had not been acknowledged by the source eNB, among the user data the UE had transmitted to the source eNB in step S60. Namely, the UE does not start user data transmission to uplink after it receives the user data transmission/reception confirmation information from the target as soon as the UE is connected to the new target eNB, but starts transmission to the uplink at the same time when the handover occurs. Such user data transmission causes a first problem as follows.
FIG. 5 shows an example of an unnecessary data transmission occurring in the handover procedure between the UE and the base station.
With reference to FIG. 5, the transmission side PDCP entity of the UE starts transmission of six user data (PDCP SDU 1 to PDCP SDU 6) to the transmission side RLC at a time T0, and the transmission RLC entity transmits user data by using RLC PDU 1 to RLC PDU 6. In this case, only RLC PDU 1, among the six RLC PDUs, is acknowledged by the reception side RLC, while the RLC PDU 2 to RLC PDU 6 have been received by the reception side RLC but their status report has not been transmitted yet. The received RLC PDU 1 to RLC PDU 6 are delivered to an upper layer at the reception side.
When handover occurs at the time T1, the transmission side and reception side RLC entities are all reset, and the transmission side PDCP entity transmits all the PDCP SDUs (PDCP SDU 2 to PDCP SDU 6) starting from the PDCP SDU 2, and the transmission side RLC entity transmits the user data by using RLC PDU A to RLC PDU G. At this time, the reception side PDCP entity transmits a PDCP status report to the transmission side PDCP entity.
At a time T2, namely, when the PDCP status report has not been received yet, the transmission side RLC entity exchanges the RLC PDUs with a peer entity (i.e., the reception side RLC entity).
Thereafter, when the PDCP status report arrives at the transmission side PDCP entity at a time T3, the transmission side PDCP entity confirms that the PDCP SDU 1 to PDCP SDU 6 are all received by the reception side, deletes the PDCP SDU 2 to PDCP SDU 6, and transmits discard indication information with respect to the discarded PDCP SDUs or the like to the RLC entity.
In this manner, although the source eNB successfully receives the PDCP SDU 1 to PDCP SDU 6 at the time T0 to T1, unless an acknowledgement of the PDCP SDU 2 to PDCP SDU 6 is transferred to the UE, the UE starts transmission of the PDCP SDU 2 to PDCP SDU 6 in the new cell. In this case, because the UE starts transmission of the PDCP SDUs before it receives the transmission/reception acknowledgement response from the target eNB, the unnecessary transmission is generated. In addition, the unnecessary retransmission of the data delays transmission of new data, negatively affecting the quality of service (QoS) of the system.